It has been discovered that the simple movement of glass articles through a tempering or annealing tunnel provided with heaters or means for the controlled cooling of glass is not satisfactory for many purposes and for a variety of reasons. For example, if the velocity of the glass is too low, the glass object may sag at high temperatures between rollers or a roller conveyor, or the glass may be nonuniformly heated. If the velocity is sufficient to avoid this problem and continuous undirectional movement is provided, then the tunnel must be extremely long. Conveyor systems which do not use rollers and maintain the same support surface in contact with the same region of the object to be transported prevent the contact zone from being heated to the same extent and in the same manner as adjacent noncontact zones.
Consequently, systems have been developed which, during the advance of the glass articles, impose a retrograde motion thereon so that, while the articles progress more or less continuously into the heat treatment chamber and emerge more or less continuously from the heat chamber, at least while the articles are in the heat treatment chamber, they undergo a back-and-forth movement which reduces the length of the chamber required for the desired degree of heat treatment while minimizing prolonged contact between the supporting elements, generally rollers, and the glass.
Roller conveyors for this purpose have been driven in various fashions and in general it has been found to be disadvantageous to utilize the simplest drive mode for the rollers of the conveyor, namely, sprockets on the ends of the support rollers and a continuous chain meshing with all of these sprockets. Hence fairly complex systems have been developed in the art to support and drive the rollers and these too have not proved to be fully satisfactory since they do not allow for effective frictional entrainment with slip control where required or provide a drive substantially without excessive play so that the retrogradal reciprocating action can be generated without imposing response lags.
The rollers structure themselves have left much to be desired since they have not generally been readily replaceable or accessible for repair, assembly or maintenance.
Within the heating chamber truly effective control of the various zones could not be asserted in spite of relatively sophisticated techniques for monitoring and regulating temperatures.
As a consequence, this field has generated a large number of patents some of which have been considered by us and which are listed below:
______________________________________ 3,867,748 3,706,544 4,332,608 4,297,121 3,338,569 4,074,805 4,300,937 3,643,789 4,133,667 4,341,546 3,608,876 4,233,053 4,332,608 4,066,430 4,364,765 3,806,312 3,994,711 4,282,026 4,314,836 4,230,475 4,364,766 4,386,952 ______________________________________
In general, while these systems attack various aspects of the problems discussed above, they have not provided a satisfactory solution to the conveyor operation, heating control and conveyor structure drawbacks mentioned previously.